Skid control system embodying automatic cycling and fail safe device

ABSTRACT

Two embodiments of vehicular skid control systems for relieving the brake pressure when an incipient skid condition is sensed. Each system includes a combined check and bleed valve that controls the flow of hydraulic actuating fluid from a master cylinder to the wheel brakes. An expansion chamber is positioned in the fluid circuit between the check and bleed valve and the wheel brakes. An ancillary source of fluid pressure, such as a power steering pump, normally holds the expansion chamber at a minimum volume condition and holds the check and bleed valve open for normal brake operation. When an incipient skid condition is sensed, the expansion chamber is permitted to increase in volume and the combination check and bleed valve is closed. Closure of the check and bleed valve isolates the master cylinder from the wheel brakes and prevents the application of further pressure to the wheel brakes. Expansion of the expansion chamber relieves the existing pressure in the wheel brakes to prevent skidding. A regulator is provided in the circuit connecting the source of ancillary fluid with the expansion chamber for maintaining at least a predetermined pressure in the circuit. The pressure maintained by the regulator is dependent upon the pressure in the brake circuit. Each embodiment includes fail safe device for providing normal brake operation in the event of failure of the source of ancillary fluid. In one embodiment a fail safe device is provided that bypasses the expansion chamber and the check and bleed valve in the event of failure of the ancillary pressure source to provide sufficient pressure. An indicator provides a visual indication to the vehicle operator when the fail safe device is operative so that the source of failure can be investigated and rectified. In the other embodiment a blocking member is provided for retaining the expansion chamber in a minimum volume position and the check and bleed valve in its opened position in the event of failure of the ancillary fluid pressure source. In each embodiment, certain of the elements are spring biased in opposition to the bias exerted by the source of ancillary fluid pressure so that these elements may be automatically cycled each time the vehicle engine is started and stopped.

United States Patent Every et a1.

[451 Oct. 21, 1975 1 SKID CONTROL SYSTEM EMBODYING AUTOMATIC CYCLING AND FAIL SAFE DEVICE [75] Inventors: Peter Every, Livonia; Leonard T.

I Tribe, Ann Arbor, both of Mich.

[731 Assignee: Kelsey-Hayes Company, Romulus,

Mich.

[22] Filed: Aug. 24, 1973 [21] Appl. No.2 391,490

[52] US. Cl. 303/21 F; 303/10 [51] Int. Cl. B60T 8/02 [581 Field of Search 303/21 F, 61-63,

[56] References Cited UNITED STATES PATENTS 3,515,440 6/1970 Every et al. 303/ 2l F 3,602,554 8/1971 Ichimura 303/21 F 3,610,701 10/1971 Riordan 303/21 F 3,672,731 6/1972 Koivunen 303/21 F 3,747,990 7/1973 Tanguy 303/21 F 3,810,680 5/1974 Schenk 303/21 F X 3,813,130 5/1974 Inada 303/21 F Primary Examiner-George E. A. I-Ialvosa Assistant Examiner-Douglas C. Butler Attorney, Agent, or FirmHarness, Dickey & Pierce [57] ABSTRACT Two embodiments of vehicular skid control systems for relieving the brake pressure when an incipient skid condition is sensed. Each system includes a combined check and bleed valve that controls the flow of hydraulic actuating fluid from a master cylinder to the wheel brakes. An expansion chamber is positioned in the fluid circuit between the check and bleed valve and the wheel brakes. An ancillary source of fluid pressure, such as a power steering pump, normally holds the expansion chamber at a minimum volume condition and holds the check and bleed valve open for normal brake operation. When an incipient skid condition is sensed, the expansion chamber is permitted to increase in volume and the combination check and bleed valveis closed. Closure of the check and bleed valve isolates the master cylinder from the wheel brakes and prevents the application of further pressure to the wheel brakes. Expansion of the expansion chamber relieves the existing pressure in the wheel brakes to prevent skidding. A regulator is provided in the circuit connecting the source of ancillary fluid with the, expansion chamber for maintaining at least a predetermined pressure in the circuit. The pressure maintained by the regulator is dependent upon the pressure in the brake circuit. Each embodiment includes fail safe device for providing normal brake operation in the event of failure of the source of ancillary fluid. In one embodiment a fail safe device is provided that bypasses the expansion chamber and the check and bleed valve in the event of failure of the ancillary pressure source to provide sufficient pressure. An indicator provides a visual indication to the vehicle operator when the'fail safe device is operative so that the source of failure can be investigated and rectified. In the other embodiment a blocking member is provided for retaining the expansion chamber in a minimum volume position and the check and bleed valve in its opened position in the event of failure of the ancillary fluid pressure source. In each embodiment, certain of the elements are spring biased in opposition to the bias exerted by the source of ancillary fluid pressure so that these elements may be automatically cycled each time the vehicle engine is started and stopped.

11 Claims, 2 Drawing Figures US. Patent 0a. 21, 1975 Sheet 2 of2 3,913,984

SKID CONTROL SYSTEM EMBODYING AUTOMATIC CYCLING AND FAIL SAFE DEVICE BACKGROUND OF THE INVENTION This invention relates to a vehicular skid control system and more particularly to a skid control system in which certain of the components are automatically cycled to insure operativeness after long periods of inactivity and to a skid control system embodying an improved fail safe device for permitting substantially normal brake operation in the event of failure of the skid control module.

Automotive skid control systems generally provide structure for periodically relieving the brake pressure when an incipient skid condition is sensed for preventing skidding. In normal vehicular operation, the skid control system may not be called upon to operate for long periods of time. If the various components of the skid control system remain in their normal braking positions for long periods of time, there is a likelihood that these components may freeze" in this position and prevent the skid control system from operating when it is called upon to do so.

It is, therefore, a principal object of this invention to provide a vehicular skid control system in which certain of the components are automatically cycled periodically between their normal braking position and a skid control position.

It is a further object of the invention to provide a skid control system in which the automatic cycling occurs each time the vehicle engine is started and stopped.

It is another object of the invention to provide a vehicular skid control system in which automatic cycling is inherent in the design each time predetermined conditions arise.

Among the movable components for vehicular skid control systems are generally included a valve that controls the communication between the master cylinder and the wheel cylinders of the controlled wheels for isolating these wheels from the master cylinder under incipient skid conditions. An expansion chamber is also incorporated between the valve and the wheel cylinders for relieving the pressure in this circuit under the incipient skid condition. If either of these components fail and move to their skid control position, normal braking operation will be impossible.

It is, therefore, another principal object of this invention to provide a skid control systemwith a fail safe device that permits substantially normal braking operation in the event of failure. V M

It is a further object of the invention to provide a fail safe device that precludes movement of components of the skid control system from a normal position to a skid control position in the event of failure.

SUMMARY or THE INVENTION A first feature of this invention is adapted to be embodied in a skid control system for a self-propelled vehicle having at least one wheel brake and a brake actuating device for selectively actuating the one wheel brake. The skid control system includes means for modifying the actuation of the one wheel brake in response to a predetermined signal to prevent a skid condition. This means includes a member that is movable between a normal braking position and a skid control position. Means are provided for automatically moving this member between these positions in response to stopping and restarting of the vehicle engine for automatically cycling the member.

Another feature of the invention is also adapted to be embodied in a skid control system for a vehicle having at least one wheel brake and a brake actuating device for selectively actuating the one wheel brake. Again skid control means modify the actuation of the one wheel brake in response to a predetermined signal to prevent the skid condition. The skid control means includes a member that is movable between a normal braking position and a skid control position and fluid pressure means urge the movable member to its normal braking position. Fail safe means preclude movement of the movable member from its normal position in the event of failure of the fluid pressure means for insuring normal brake operation in the event of such failure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates, in part schematically, a skid control system incorporating a first embodiment of this invention.

FIG. 2 is a partially schematic illustration of a; skid control system incorporating a second embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment of FIG. 1

A vehicle braking system including a skid control system incorporating a first embodiment of this invention is identified generally by the reference numeral 11. The braking system includes a master cylinder, of the dual type, 12 having independent output circuits 13 and 14. The circuit 13 serves the brake cylinders 15 and 16 of the front wheel brakes, which may be of the disk or drum type. The circuit 14 serves the wheel cylinders 17 and 18 of the rear wheel brakes via a skid control modulator, indicated generally be the reference numeral 19, and conduit 21. The rear wheel brakes also may be of either the drum or disk type. In the illustrated embodiment, the skid control modulator 19 acts only on the brake circuit serving the rear wheels and controls both of the wheel cylinders 17 and 18 simultaneously. It is to be understood that it is within the purview of the invention to provide a similar skid control modulator for the front wheel brakes to to provide independent control for each wheel cylinder 15, 16, 17 and 18. Such modifications are deemed to be within the scope of those skilled in the art and for that reason will not be described.

Wheel speed sensors 22 and 23 are associated with each of the rear wheels and provide electric signals via conduits 24 and 25 to an electrical module 26. In the event of an incipient skid condition, which may be sensed in any of a variety of known manners, the module 26 provides a signal to the modulator 19 via a conductor indicated schematically at 27.

Referring now specifically to the modulator 19, the master cylinder conduit 14 delivers fluid to a chamber 28 in which a combined check and bleed valve, indicated generally by the reference numeral 29, is positioned. The check and bleed valve 29 is of the construction described in U.S. Pat. No. 3,486,800, entitled Skid-Control System Including Hydraulic Modulating Valve, issued Dec. 30,, 1969 in the name of David T. Ayers, Jr. Reference may be hadto that patent for a description of the complete structure and operation of the valve 29. Basically, the valve 29 includes a first plunger 31' that directly actuates a valve 32 for controlling the flow from the chamber 28 into a conduit 33. A second plunger 34 is slidably supported by the first plunger 31 and has a head 35 that is movable into engagement with a portion 36 for controlling the flow through a restricted path from the chamber 28 into the conduit 33. The plunger 34 is longer than the plunger 31 so that on reapplication of the brakes, as will become more apparent, initial flow from the master cylinder 12 to the wheel cylinders 17 and 18 will take place through this restricted path.

At the mouth of the conduit 33 an enlarged diameter chamber 36 is provided. The chamber 36 has slidably supported Within it an expansion chamber plunger 37. In the normal braking position, as shown in the figure, the plunger 37 is held at a minimum volume condition and engages the plungers 31 and 34 to hold the combination check and bleed valve in a fully open condition against the bias of valve springs 38 and 39.

At the base of the chamber 36 a further, larger diameter chamber 41 is provided. Within the chamber 41 is slidably supported a power piston 42. Power piston 42 is formed with a concentric bore 43. Slidable within the bore 43 is a cylindrical extension 44 of the expansion chamber plunger 37. Facing shoulders 45 and 46 of the expansion chamber plunger 37 and power piston 42 are normally held in abutment. A coil compression spring 47 encircles an extension 48 of the power piston 42 and normally urges the piston 42 toward the left as viewed in this figure.

A source of ancillary fluid pressure is provided for maintaining the power piston 42, expansion chamber plunger 37 and check and bleed valve 29 in the normal braking position, as shown in the figure, in opposition to the action of the springs 47, 38 and 39. The term ancillary fluid refers herein to a fluid pressure source other than the brake pressure. In the illustrated embodiment, this ancillary fluid pressure source is provided by an engine driven pump 49 which may be the vehicle power steering pump. The pump 49 draws fluid from a sump 51 through a conduit 52 and discharges it under pressure into a conduit 53. The pressure in the conduit 53 is controlled by a regulator, indicated generally be the reference numeral 54, in a manner which will become more apparent as this description proceeds.

The conduit 53 serves the power steering gear of the vehicle, indicated schematically at 55, via the regulator 54 and a conduit 56. A conduit 57 returns the fluid from the power steering gear 55 to the sump 51.

A valve, indicated generally by the reference numeral 58 controls the flow of the ancillary fluid from the conduit 53 to the power piston chamber .41. The valve 58 is fed via a conduit 59 which terminates at a valve seat 61. juxtaposed to the valve seat 61 is a valve plate 62 that is normally biased by a spring 63 to an opened position. In this position, the power steering fluid may flow from the conduit 59 into a valve chamber 64. The valve chamber is ported, as at 65, to the chamber 41.

A solenoid winding 66 is positioned in the valve housing and, when actuated, draws the valve plate 62 downwardly into a closed relationship with the valve seat 61. Simultaneously, a valve seat 67 that is directly. opposite to the valve seat 61 is opened. The power steering fluid may then be displaced, in a manner to be described, from the chamber 41, port 65 and chamber 64 back to the sump 51 via a return conduit 68.

The regulator 54 includes a plunger 71 having a portion that is slidably supported in a bore 72. The plunger 71 has a projection 73 that cooperates with a counterbore 74 that terminates in the conduit 56 so as to regulate the pressure drop and, accordingly, the pressure in the conduit 53.

A coil spring 76 encircles an extension 77 of the plunger 71 to provide a preselected bias on the plunger 71. In addition, a second plunger 78 has a bore 79 that receives a projection 81 of the plunger portion 77. The second plunger 78 is supported in a bore 82 that experiences the pressure in the braking circuit and particularly that in the conduit 14 through a conduit 83. As the brake pressure from the master cylinder to the rear wheel brakes is increased, the plunger 78 will bias the plunger 71 to a direction that increases the pressure in the conduit 53.

As is common with many forms of skid control systems, actuation of the skid control modulator 19 causes the check and bleed valve 29 to close and isolate the rear wheel brake cylinders 17 and 18 from the master cylinder supply conduit 14. At the same time, the expansion chamber plunger 37 is permitted to regress against the bias of the power piston 42 to relieve the pressure in the rear wheel brake cylinders 17 and 18. The source of ancillary fluid, the power steering pump 49, is utilized to hold the check and bleed valve 29 open and to hold the expansion chamber plunger 37 in its minimum volume condition. In the event of failure of the power steering pump 49 or bursting of any of the conduits 53, 59 or 56, a fail safedevice, indicated generally by the reference numeral 84, is provided for delivering the brake fluid directly from the master cylinder conduit 14 to the rear wheel brake cylinders 17 and 18 independently of the chamber 36. This will prevent the loss of actuating fluid and will permit normal brake operation albeit without the skid control function.

The fail safe device 84 includes an isolation valve 85 that is carried by a plunger 86. The plunger 86 has a bypass valve end portion 87 that controls the flow through a port 88 formed at the end of a conduit 89. The conduit 89 extends from the chamber 28 to a chamber 91 in which the bypass valve 85 is located. A coil spring 92 encircles the plunger 86 and normally biases the isolation valve 85 toward a closed position, with a port which will be described, and the plunger end portion 87 to an open position.

The plunger 86 extends through a transverse passage 93 that interconnects the chamber 91 with a conduit 94 that extends to the expansion chamber 36. The isolation valve 85 normally controls the flow through the conduit 94 and passage 93. When this passage is open, brake fluid can flow from the master cylinder past the check and bleed valve 29 through the expansion chamber 36 and on to the chamber 91 via conduit 94 and passage 93. The brake fluid is then delivered to the conduit 21 via a conduit 95.

The isolation valve 85 is normally held in its opened position by means including a plunger 96 that is slidably supported in a stepped bore 97 and which has an end portion 98 that engages the plunger 86. The plunger 96 has its opposite end bored as at 99 to receive a cylindrical extension 101 of a fail safe piston 102. The piston 102 is slidably supported in a bore 103 that experiences the pressure in the conduit 53 via a conduit 104. A coil spring 105 encircles the plunger 96 and bears against a shoulder 106 to act in opposition to the pressure in the bore 103.

The plunger 96 is formed with an annular relief 107 that terminates at one end in an inclined ramp portion 108. The actuating plunger 109 of a switch 111 is received within the relief 107 and is adapted to be actuated by the inclined surface 108, as will become apparent.

OPERATION OF EMBODIMENT OF FIG. 1

As has been noted, this figure illustrates the system as it appears when in condition for normal brake operation. The power steering pump 49 is generating pressure in the conduit 53 at an amount determined by the position of the regulator 54 which is determined in this condition by the preload on the spring 76. The predetermined pressure will be maintained regardless of the demands of the power steering gear 55. This pressure is transmitted through the conduit 59 and normally open valve 58 to the chamber 41 to hold the power piston 42 in a position so as to maintain a minimum volume in the chamber 36 and to hold the check and bleed valve 29 in its open position. Simultaneously, the pump pressure is .transmitted through the conduit 104 to' act on the fail safe piston 102 and hold the isolation valve 85in its opened position and the bypass valve 87 in its closed position.

Upon actuation of the master cylinder 12, pressure will be exerted through the circuit 13 to the front wheel brake cylinders 15 and 16. Pressure will be transmitted through the conduit 14 to the chamber 28. This fluid flows past the open check valve 32, through the passage 33 into the expansion chamber 36. The pressure acting on the power piston 42 holds the expansion chamber plunger 37 in the illustrated position and brake fluid passes from the chamber 36 into the conduit 94. From the conduit 94 brake fluid flows through passage 93 to chamber 91 past the open isolation valve 85. Fluid leaves the chamber 91, passes to conduit 95 and thence to conduit 21 to actuate the rear wheel cylinders 17 and 18.

The master cylinder pressure is also transmitted from the chamber 28 through the conduit 83 on to the smaller plunger 78 of the regulator 54. This will create a bias on the plunger 71 to the left to restrict the flow through the counterbore 74 and raise the pressure in the conduit 53 in proportion to the master cylinder pressure exerted on the conduit 14. Hence, further pressure will be exerted on the power piston 42 to resist movement of the expansion chamber plunger 37 and to resist closure of the check and bleed valve 29. Because of this regulation in proportion to master cylinder pressure, it is possible to actuate the system at a lower net pressure. Said another way, if the regulator 54 was not responsive to the pressure in the brake conduit 14, the power piston 42 and pressure in the conduit 53 would have to be set sufficiently to overcome the maximum anticipated pressure in the conduit 14. As a result, it would not be possible to obtain as smoothly a modulated operation.

It should be noted that the ratio of the area of the plunger 71 exposed to the pressure in the chamber 72 to the area of the plunger 78 exposed to the pressure in the chamber 82 is less than the corresponding ratio of the area of the power piston 42 exposed to the pressure in the chamber 41 to the area of the expansion chamber plunger 37 exposed to the pressure in the expansion chamber 36. Thus, as the pressure increases in the brake conduit 14, the regulator 54 will maintain an 5 increased pressure on the power piston 42 and on the expansion chamber plunger 37 somewhat in excess of the pressure needed to maintain the check and bleed valve 29 in the opened position.

In the event an incipient skid condition is sensed by the electric module 26, a signal will be sent via the conductor 27 to the valve 58. The solenoid winding 66 will be energized to draw the valve plate 62 downwardly against the bias of the spring 63. The valve seat 61 will then be closed to isolate the power piston chamber 41 from the pump supply conduit 53. Simultaneously, the pressure in the chamber 41 will be dumped to the sump 51 via the open valve seat 67 and conduit 68. the spring 47 and brake pressure on the plunger 37 will then urge the power piston 42 to the left. The springs 38 and 39 will, at this time, close the combined check and bleed valve 29 cutting off the communication between the master cylinder conduit 14 and the wheel cylinder conduit 21. The pressure acting in the expansion chamber 36 will, at this time, be sufflcient to cause the expansion chamber plunger 37 to move to the left and increase the volume in the expansion chamber 36. This increase in volume relieves the pressure on the wheel cylinders 17 and 18 and, in effect, will release the brakes. It should be noted that at this time the expansion chamber plunger 37 will have moved away from the plungers 33 and 34 of the check and bleed valve 29.

At a predetermined condition, the electric module 26 will send a signal to cause reapplication of the brakes. In the illustrated embodiment this is done by discontinuing the electric current flow to the solenoid winding 66. The valve plate 62 is then urged upwardly by the spring 63 to close the valve seat 67 and open the valve seat 61. Power steering pump pressure will then be reapplied to the power piston 42 which, in turn, urges the expansion chamber plunger 37 to the right to decrease the volume in the expansion chamber 36. At this time, the combined check and bleed valve 29 will still be closed. Thus, the wheel cylinders 17 and 18 are gradually reapplied first by the action of the expansion chamber plunger 37. It should be noted that, unless the operator has removed his foot from the brake pedal, wheel master cylinder pressure will still be experienced in the conduit 14 and chamber 28.

After the expansion chamber plunger 37 has moved sufficiently it will engage the bleed valve plunger 34 to open it from its engagement with the seat 36. Master cylinder pressure will, then, be similarly reapplied through the small clearance volume. Upon continued movement, the plunger 31 will be engaged to open the check valve 32 and permit free communication between the conduit 14 and the wheel cylinders 17 and 18.

It will be noted that pressure of the power steering pump 49 is required to hold the check and bleed valve 29 and expansion chamber plunger 37 in their normal braking positions. In the event of failure of the pump 49 or rupturing of any of the conduits 53, 56, 59, 104 or other fluid leakages, there will be insufficient pressure to resist the brake pressure that tends to cause the checkand bleed valve 29 and expansion chamber plunger 37 to move to their skid control positions. However, under this circumstance there will be insufficient pressure acting on the fail safe piston 102 to resist the action of the springs 105 and 92. Thus, the plungers 96 and 86 will shuttle to the left. This movement will first cause opening of the bypass valve 87 and the communication of the conduit 89 with the chamber 91 and subsequently closing of the isolation valve 85. Thus, a bypass conduit is open to permit brake pressure to be exerted from the master cylinder to the wheel cylinders 17 and 18. The brake pressure flows from the conduit 14 and chamber 28 into the conduit 89. This fluid can then flow through the conduit 95 and conduit 21 to the brake cylinders 17 and 18 to effect actuation. This is accomplished without significant loss of actuating fluid. Some small amount of fluid will be lost, however, due to the movement of the check and bleed valve 29 to its closed position. This amount of fluid is relatively insignificant.

At this time the plunger 96 shuttles to the left, the switch 111 will be actuated to provide a visual warning via a suitable circuit and indicator either a light or buzzer (not shown) to the vehicle operator. He will then be warned that his skid control system is not operating properly. Upon re-establishment of satisfactory pressure from the pump 49, the fail safe valve 84 will be automatically returned to the position shown in the figure.

In vehicular operation the skid control modulator 19 may not be called upon to function for long periods of time. That is, an incipient skid condition may not occur for a considerable time period. It is, therefore, desirable to periodically cycle certain of the elements to insure that they will not stick or otherwise bind in their normal positions. The construction of certain elements of the modulator 19 is such that this automatic cycling will occur each time the vehicle engine is started and stopped. As has been noted, the pump 49 is driven by the vehicle engine. Thus, each time the engine stops the pump 49 will not exert pressure in the conduits 53, 59 and 104. Accordingly, the spring 76 will move the regulator plunger 79 to an extreme left-hand position each time the engine stops. In a like manner, the-spring 47 will urge the power piston 42 to an extreme left-hand position. The spring 105 will cause a similar action on the plunger 96 and piston 102. The spring 92 will also move the fail safe valve 84 to its closed position and the associated plunger to its opened position. Upon restarting of the vehicle engine, each one of the aforenoted elements will be moved back to the normal braking positions.

The ratio of the area of the fail safe plunger 102 exposed to the pressure in the chamber 103 to the portion exposed to the pressure at the termination of the conduit 94 is less than the corresponding ratio of areas of the power piston 42 and expansion chamber plunger 37. Thus, in the event of pressure failure the fail safe device will shuttle before the expansion chamber plunger. The aforementioned area ratio of the fail safe device is greater than the corresponding ratio of the area of the regulator 54.

EMBODIMENT OF FIG. 2

In the previously described embodiment an arrangement was provided for bypassing the flow of brake fluid from the master cylinder to the wheel cylinders in the event of failure of the skid control module, specifically in the event of failure of the power steering pump to provide sufficient pressureto hold the expansion chamher plunger and bleed and check valves in their normal braking positions. In this embodiment, a different form of fail safe device is provided. The main components of the skid control module and of the braking system are the same. For this reason, components which are the same as in the previously described embodiment, have been identified by the same reference numerals and their description will not be repeated.

In this embodiment a blocking piston, indicated generally by the reference numeral is supported for re-. ciprocation in a bore 126 that extends transversely to the chamber 41 and immediately adjacent the position the head of the power piston 42 occupies when the power piston 42 is in its normal braking condition. A spring 127 normally urges a reduced diameter portion 128 of the blocking piston 125 toward a position within the chamber 41 but slightly spaced from the power piston 42. The lower end of the bore 126 is vented to the sump 51 via a conduit 129 and the conduit 57. As long as the pressure of the pump 49 is experienced in the power piston chamber 41, the blocking piston 125 will be held in a retracted position against the bias of the spring 127 as shown in FIG. 2.

In this embodiment, the expansion chamber 36 is in direct commmunication with the rear brake conduit 21 via a conduit 131. Thus, during normal brake operation the rear wheel cylinders 17 and 18 receive fluid under pressure via the conduits 21, 131, passage 33, chamber 1 28 and conduit 14.

The operation of the skid control module during the normal skid control cycle is the same as in the previously described embodiment. For this reason, this operation will not be described again in detail.

When the valve 58 is operated to dump the chamber 41 the pressure tending to hold the blocking piston 125 in its retracted position will also be relieved. However, the components are arranged in such a way that the expansion chamber plunger 37 will move to increase the volume in the expansion chamber 36 and the power.

piston 42 will be driven thereby to a position along the chamber 41 in registry with the blocking piston 125. That is, under normal skid control operation, the power piston 42 will retract faster than the blocking piston 125 can move to its blocking position. Hence, normal skid control operation may be effected. In the event of failure of the pump 49 to deliver sufficient pressure through the conduit 53 to the power piston chamber 41, the spring 127 will urge the blocking piston 125 to an exposed position. In this position the blocking piston 125 will prevent any substantial movement of the power piston -42. At the same time, the expansion chamber plunger 37 will be heldagainst substantial.

movement as will the combined check and bleed valve 29. Thus, each of these components is held substan: tially in a normal braking position in the event of hydraulic failure. Normal brake operation is thus possible draulically assisted brake system, the regulator may be eliminated. In such an embodiment, pressure in the conduit 53 would be generated directly by the hydraulic booster of the brake system. Various other changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.

We claim:

1. A skid control system for a self-propelled vehicle having at least one hydraulically operated wheel brake comprising a brake actuating device including a hydraulic circuit for selectively actuating the one wheel brake, skid control means including a valve for controlling the flow through said hydraulic circuit for modifying the actuation of the one wheel brake in response to a predetermined signal to prevent a skid condition, said valve being movable between an opened position and a closed position, a member movable between a normal braking position and a skid control position, said member being operatively connected with said valve and being effective when in its normal braking position for holding said valve in its opened position, said valve being movable to its closed position when said member moves from its normal braking position toward its skid control position but before said member reaches its skid control position, and means for moving said member between said positions in response to the stopping and restarting of the vehicle engine for automatically cycling said member.

2. A skid control system as set forth in claim 1 wherein the brake actuating device includes a hydraulic circuit for actuating the one wheel brake and the movable member is operatively connected to a wall of a variable volume chamber, said wall being movable to increase the volume of said chamber under the predetermined signal to relieve the hydraulic pressure on the one wheel brake.

3. A skid control system as set forth in claim 2 further including fail safe means for delivering actuating fluid to the one wheel brake in the event of a failure of said skid control means for providing a brake operation.

4. A skid control system as set forth in claim 3 wherein the fail safe means includes a bypass circuit and normally closed bypass valve means for controlling the flow through said bypass circuit for delivering fail safe actuating fluid to the one wheel brake independently of the variable volume chamber.

5. A skid control system as set forth in claim 3 wherein the fail safe means includes blocking means for precluding movement of the wall in the event of failure.

6. A skid control system as set forth in claim 5 wherein the skid control means further includes a source of ancillary fluid under pressure for operating the means for increasing the volume in the variable volume chamber, said blocking means being responsive to the pressure generated bythe source of ancillary fluid.

7. A skid control system as set forth in claim 6 wherein the ancillary fluid normally holds the means for increasing the volume in the variable volume chamber in a minimum volume condition and the blocking means in a released position in which the wall is free to move.

8. A skid control system as set forth in claim 1 wherein the last named means includes means driven by the engine for moving the member from one of its positions to the other of its positions.

9. A skid control system as set forth in claim 8 wherein the means driven by the engine moves the member to its normal braking position.

10. A skid control system as set forth in claim 8 further including spring means for urging the member to its other position.

11. A skid control system as set forth in claim 10 wherein the means driven by the engine moves the member to its normal braking position. 

1. A skid control system for a self-propelled vehicle having at least one hydraulically operated wheel brake comprising a brake actuating device including a hydraulic circuit for selectively actuating the one wheel brake, skid control means including a valve for controlling the flow through said hydraulic circuit for modifying the actuation of the one wheel brake in response to a predetermined signal to prevent a skid condition, said valve being movable between an opened position and a closed position, a member movable between a normal braking position and a skid control position, said member being operatively connected with said valve and being effective when in its normal braking position for holding said valve in its opened position, said valve being movable to its closed position when said member moves from its normal braking position toward its skid control position but before said member reaches its skid control position, and means for moving said member between said positions in response to the stopping and restarting of the vehicle engine for automatically cycling said member.
 2. A skid control system as set forth in claim 1 wherein the brake actuating device includes a hydraulic circuit for actuating the one wheel brake and the movable member is operatively connected to a wall of a variable volume chamber, said wall being movable to increase the volume of said chamber under the predetermined signal to relieve the hydraulic pressure on the one wheel brake.
 3. A skid control system as set forth in claim 2 further including fail safe means for delivering actuating fluid to the one wheel brake in tHe event of a failure of said skid control means for providing a brake operation.
 4. A skid control system as set forth in claim 3 wherein the fail safe means includes a bypass circuit and normally closed bypass valve means for controlling the flow through said bypass circuit for delivering fail safe actuating fluid to the one wheel brake independently of the variable volume chamber.
 5. A skid control system as set forth in claim 3 wherein the fail safe means includes blocking means for precluding movement of the wall in the event of failure.
 6. A skid control system as set forth in claim 5 wherein the skid control means further includes a source of ancillary fluid under pressure for operating the means for increasing the volume in the variable volume chamber, said blocking means being responsive to the pressure generated by the source of ancillary fluid.
 7. A skid control system as set forth in claim 6 wherein the ancillary fluid normally holds the means for increasing the volume in the variable volume chamber in a minimum volume condition and the blocking means in a released position in which the wall is free to move.
 8. A skid control system as set forth in claim 1 wherein the last named means includes means driven by the engine for moving the member from one of its positions to the other of its positions.
 9. A skid control system as set forth in claim 8 wherein the means driven by the engine moves the member to its normal braking position.
 10. A skid control system as set forth in claim 8 further including spring means for urging the member to its other position.
 11. A skid control system as set forth in claim 10 wherein the means driven by the engine moves the member to its normal braking position. 